1. Field of the Invention
The present invention relates to a negative pressure type booster for use in a vehicular brake system, and more particularly to a negative pressure type booster for making up for lack of force at a brake pedal so as to assist a driver during emergency braking.
2. Description of the Related Art
Recent analyses have revealed that, in many cases, a vehicle fails to exhibit its braking capability sufficiently when a driver steps on a brake pedal in alarm upon sudden emergence of an obstacle in the path of the vehicle; i.e., during emergency braking, since a force that the driver applies to a brake pedal is considerably smaller than a force that generates such a high brake fluid pressure as to cause wheel lock-up. On the basis of the analytic results, there have been proposed and put into practical use devices that judge from the speed of stepping on a brake pedal or the rate of increase of the fluid pressure of a master cylinder whether or not braking is of an ordinary operation or an emergency operation, and, during emergency braking, automatically increase brake fluid pressure to such a level as to cause wheel lock-up.
WO 01/32488 discloses one of such negative pressure type boosters of various structures. The disclosed negative pressure type booster can change over its input-output characteristic between an ordinary braking characteristic and an emergency braking characteristic. During emergency braking, the input-output characteristic is changed over from the ordinary braking characteristic to the emergency braking characteristic, thereby generating a strong braking force.
The negative pressure type booster disclosed in WO 01/32488 includes a housing having a pressure chamber formed therein; a movable diaphragm mounted in the housing in a frontward/rearward movable condition and dividing the pressure chamber into a constant-pressure chamber and a variable-pressure chamber; a power piston connected to the movable diaphragm; an input member provided in the power piston in a frontward/rearward movable condition relative to the power piston and receiving an external operation force; an output member for outputting, to an external device, a thrust force generated by the power piston; a slide valve disposed between the power piston and the input member, and mounted coaxially to the power piston in a frontward/rearward movable condition; holding means for holding the slide valve at a predetermined frontward position when the frontward-movement quantity of the input member relative to the power piston is equal to or less than a predetermined value; moving means for moving the slide valve rearward by a predetermined quantity when the frontward-movement quantity of the input member relative to the power piston is greater than the predetermined value; and return means for returning the slide valve to the predetermined frontward position when the power piston and the slide valve return to respectively predetermined positions relative to the housing. The disclosed negative pressure type booster also includes a control valve mounted in the power piston and including an atmospheric control valve portion for establishing/shutting off communication between the variable-pressure chamber and the atmosphere in cooperation with an atmospheric valve seat provided on the input member, and a vacuum control valve portion for establishing/shutting off communication between the variable-pressure chamber and the constant-pressure chamber in cooperation with a vacuum valve seat provided on the power piston and/or a vacuum valve seat provided on the slide valve. The disclosed negative pressure type booster provides various advantages. Notably, the term “rearward” refers to the side toward a brake pedal or the side toward the vehicular rear as viewed from the negative pressure type booster, and the term “frontward” refers to the side toward a brake master cylinder or the side toward the vehicular front as viewed from the negative pressure type booster.
However, in the conventional negative pressure type booster, a holding member, which partially constitutes the holding means, the movable means, and the return means, is mounted to the power piston via a fulcrum member in a radially tiltable condition. In other words, the fulcrum member is required in mounting the holding member to the power piston. Also, the holding member is elongated in the axial direction of the power piston and extends along the axial direction, thus causing an increase in the overall length (axial length) of the negative pressure type booster. The holding member is disengaged from the slide valve by means of tilting radially outward about its portion of engagement with the fulcrum member. Thus, accuracy in disengagement timing decreases, resulting in a tendency toward a variation in a threshold value for effecting an emergency braking characteristic. Therefore, activation of emergency braking tends to become inconsistent.